1. Field of the Invention
The present invention relates generally to semiconductor memory devices and, more particularly, to an open digit line architecture memory device having a balanced sense amplifier control.
2. Description of the Related Art
An increasing number of electronic equipment and electronic-based systems require some form of high-speed memory devices for storing and retrieving information (or xe2x80x9cdataxe2x80x9d). While the types of such memory devices vary widely, semiconductor memory devices are most commonly used in memory applications requiring implementation in a relatively small area. Within this class of semiconductor memory devices, the DRAM (Dynamic Random Access Memory) is one of the more commonly used types.
The DRAM has memory arrays consisting of a number of intersecting row and column lines of individual transistors or memory cells. In a conventional dynamic random access memory (DRAM) device each memory cell, or memory bit, consists of one transistor and one capacitor. A terminal of the transistor is connected to a digit line, or bitline, of the memory device. Another terminal of the transistor is connected to a terminal of the capacitor and the gate terminal of the transistor is connected to a wordline of the memory device. The transistor thus acts as a gate between the digit line and the capacitor.
The second terminal of the capacitor is connected to a voltage rail which carries a voltage, such as VCC/2. Thus, when the wordline for a particular cell is active, the gate transistor is in a conducting state and the capacitor is connected to the digit line. The capacitor stores a charge that, depending on whether the polarity of the voltage across the capacitor is positive or negative, represents either a logic high or a logic low value.
Typically, a microcomputer circuit selects (or activates) particular row and column lines to access selected memory cells. xe2x80x9cAccessxe2x80x9d typically refers to reading data from or writing data to selected memory cells. Reading data from the memory cells involves the use of a sense amplifier to detect whether the voltage level stored in the memory cell represents a binary one or a binary zero.
Memory devices are typically constructed with complementary digit lines of equal capacitance. Sense amplifiers are connected between the digit lines and operate to sense the differential voltage across the digit lines. An open digit line architecture, as illustrated in FIG. 1, features the sense amplifier circuits 10 between arrays 12, 14, 16, 18. True and complement digit lines, such as for example D120 and D1*22 come from separate arrays 14, 16 on each side of the sense amplifiers 10 as illustrated in FIG. 1.
Operation of the sense amplifiers 10 is accomplished by applying various signals to each sense amplifier to fire the sense amplifiers as is well known in the art. FIG. 2 illustrates the circuitry of a sense amplifier 10 of FIG. 1. As is generally known in the art, the term sense amplifier includes a collection of circuit elements connected to the digit lines of a DRAM array. This collection typically includes devices for equilibration and bias, one or more N-sense amplifiers, one or more P-sense amplifiers, and devices connecting selected digit lines to input/output signal lines as will be described below.
As shown in FIG. 2, sense amplifier 10 includes a P-sense amplifier 30 and an N-sense amplifier 40 for sensing charge stored in the selected memory cell of the selected array via a voltage differential on the pair of digit lines D120 and D1*22. Equilibration circuit 50 is provided to equilibrate the digit lines D120 and D1*22. Equilibration circuit 50 includes transistor 52 with a first source/drain region coupled to digit line D120, a second source/drain region coupled to digit line D1*22 and a gate coupled to receive an equilibration signal EQ. Equilibration circuit 50 further includes first and second transistors 54 and 56. Transistor 54 includes a first source/drain region that is coupled to digit line D120, a gate that is coupled to receive the equilibration signal EQ and a second source/drain region that is coupled to receive an equilibration voltage Veq, which is typically equal to Vcc/2. Second transistor 56 includes a first source/drain region that is coupled to digit line D1*22, a gate that is coupled to receive the equilibration signal EQ and a second source/drain region that is coupled to the equilibration voltage Veq. When the signal EQ is at a high logic level, equilibration circuit 50 effectively shorts digit line D120 to digit line D1*22 such that both lines are equilibrated to the voltage Veq.
When P-sense amplifier 30 and N-sense amplifier 40 have sensed the differential voltage across the digit lines D120 and D1*22 (as described below), a signal representing the charge stored in the accessed memory cell is output from the DRAM device on the input/output (I/O) lines I/O 36 and I/O*38 by connecting the I/O lines I/O 36 and I/O*38 to the digit lines D120 and D1*22, respectively. A column select (CSEL) signal is applied to transistors 40, 42 to turn them on and connect the digit lines D120 and D1*22 to the I/O lines I/O 36 and I/O*38.
The operation of the P-sense amplifier 30 and N-sense amplifier 40 is as follows. These amplifiers work together to detect the access signal voltage and drive the digit lines D120 and D1*22 to Vcc and ground accordingly. As shown in FIG. 2, the N-sense amplifier 40 consists of cross-coupled NMOS transistors 42, 44 and drives the low potential digit line to ground. Similarly, the P-sense amplifier 30 consists of cross-coupled PMOS transistors 32, 34 and drives the high potential digit line to Vcc. The NMOS pair 42, 44 or N-sense-amp common node is labeled RNL*. Similarly, the P-sense-amp 30 common node is labeled ACT (for ACTive pull-up). Initially, RNL* is biased to Vcc/2 and ACT is biased to ground. Since the digit line pair D120 and D1*22 are both initially at Vcc/2 volts, the N-sense-amp transistors 42, 44 remain off due to zero Vgs potential. Similarly, both P-sense-amp transistors 32, 34 remain off due to their positive Vgs potential. A signal voltage develops between the digit line pair 20, 22 when the memory cell access occurs. While one digit line contains charge from the cell access, the other digit line serves as a reference for the sensing operation. The sense amplifier firing generally occurs sequentially rather than concurrently. The N-sense-amp 40 fires first and the P-sense-amp 30 second. The N-sense amplifier is fired by providing a signal, labeled NSA to a transistor 46 connecting the common node of the N-sense amplifier to ground. Dropping the RNL*signal toward ground will fire the N-sense-amp 40. As the voltage between RNL* and the digit lines approaches Vt, the NMOS transistor whose gate connection is to the higher voltage digit line will begin to conduct. Conduction results in the discharge of the low voltage digit line toward the RNL* voltage. Ultimately, RNL* will reach ground, bringing the digit line with it. Note that the other NMOS transistor will not conduct since its gate voltage derives from the low voltage digit line, which is discharging toward ground.
Shortly after the N-sense-amp 40 fires, ACT will be driven toward Vcc by applying a low signal PSA to PMOS transistor 48, thus connecting the common node of the P-sense amplifier 30 to Vcc. This activates the P-sense-amp 30 that operates in a complementary fashion to the N-sense-amp 40. With the low voltage digit line approaching ground, a strong signal exists to drive the appropriate PMOS transistor into conduction. This will charge the high voltage digit line toward Vcc, ultimately reaching Vcc. Since the memory bit transistor remains on during sensing, the memory bit capacitor will charge to the RNL* or ACT voltage level. The voltage, and hence charge, which the memory bit capacitor held prior to accessing will restore a full level, i.e., Vcc for a logic one and GND for a logic zero.
In an open digit line architecture as illustrated in FIG. 1, sense amplifiers on each side of an array, or section, are fired. Typically, as illustrated in FIG. 3, a global signal, such as for example the EQ, NSA or PSA signal, is driven across the memory device by a driver 60 and input to a NAND gate 62 with a section signal, supplied by a signal source such as a buffer amplifier 61, to fire the sense amps on each side of a specific section. For example, as illustrated in FIG. 3, if a row was fired in array 14, the sense amps would fire on both the left and right side of array 14. Problems exist, however, due to the signal noise inherent in an open digit line architecture (due to various coupling effects) and signal propagation of the global signal. For example, the signal propagation causes the left side to fire slightly before the right side. This slight difference in firing time can cause a margin imbalance on one side of the section as opposed to the other side of the section. That is, the side that fires last will have a reduced signal sensitivity margin for sensing data which can lead to erroneous reading of the data signal. Ideally both sense amps on each side should fire simultaneously.
Thus, there exists a need for an open digit line architecture in which the sense amplifiers on each side of a section are fired simultaneously, thus eliminating margin imbalance on one side of the section as opposed to the other side of the section.
The present invention overcomes the problems associated with the prior art and provides a method and apparatus for simultaneously firing the sense amplifiers on each side of a section, thereby significantly reducing any margin imbalance between the two sides.
In accordance with the present invention, firing of the sense amplifiers on each side of a section is controlled by a two stage NAND gate logic circuit that utilizes a tree routing scheme. By gating the global signal with a section signal through the two stage NAND gate logic circuit, the sense amplifiers on each side of a section can be fired simultaneously.
These and other advantages and features of the invention will become more readily apparent from the following detailed description of the invention which is provided in connection with the accompanying drawings.